Canadian tennis star Eugenie Bouchard won the first battle in her US Open slip-and-fall lawsuit on Thursday, when a federal jury found tournament managers primarily liable for her 2015 career-damaging accident.

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Bouchard, once ranked No. 5 in the world, but now 116th, says she suffered a concussion and had to cut short her performance in the 2015 tournament after slipping backward on a training room floor that was slick with cleaning chemicals.

It took a Brooklyn federal jury just under an hour Thursday to decide that the United States Tennis Association, which runs the Open and the US Tennis Center, is 75 percent liable for Bouchard’s fall.

Bouchard, who had entered the darkened training room at 11 p.m. — after a late mixed double match followed by press interviews — bears 25 percent responsibility for falling, the jury found.

USTA lawyers had argued that management believed all the players had left, and was only doing its job in cleaning the floor.

On Friday, the same Brooklyn federal jury will reconvene to hear testimony in the damages phase of the trial.

The jury will then decide how much Bouchard’s concussion cost her in medical expenses and lost winnings and sponsorships; the USTA would then be responsible for 75 percent of that amount.

“I’m not going to comment on anything right now, thank you,” the 23-year-old pro said after Thursday’s victory.

But her lawyer, Benedict Morelli, said he’d been confident of a win, but was surprised by the four-woman, three-man jury’s speed.

“When you have a 75 percent or better [liability verdict] in a case like that, you can’t complain about it,” he added.

Bouchard took the stand Wednesday to describe entering the darkened training room in a sports bra and shorts in hopes of a post-match ice bath.

“I screamed, ‘Oh my God, it burns!” she told jurors of feeling the chemicals against her bare back.

She will likely return to the witness stand to describe the effects of her concussion.